1. Field of the Invention
The present invention relates to the hydroxylation of phenolic compounds and, more especially, to the hydroxylation of phenols and phenol ethers by reacting same with hydrogen peroxide in the presence of a sulfonated polymer/ketone catalyst system.
2. Description of the Prior Art
Serious need exists in this art, for example for a selective process for the hydroxylation of phenol into hydroquinone and pyrocatechol that predominantly favors the production of hydroquinone.
The processes presently known to this art predominantly favor the production of pyrocatechol.
It is recognized that, in order to respond to the fluctuating requirements of the commercial market, an industrial process would be desirable that increased the production of hydroquinone relative to the amount of pyrocatechol.
Also, those processes currently in use entail conducting the hydroxylation via homogeneous catalysis.
For example, FR-A-2,071,464 describes a very important industrial process for the hydroxylation of phenols and phenol ethers.
Such process includes carrying out the hydroxylation, using hydrogen peroxide, in the presence of a strong acid. Among these strong acids, sulfuric acid, para-toluene sulfonic acid and perchloric acid are the most widely used.
The hydroxylation of phenol conducted under the operating conditions described produces a mixture of hydroquinone and pyrocatechol, with a predominance of the latter, since the hydroquinone/pyrocatechol ratio typically ranges from 0.3 to 0.7.
Pyrocatechol is thus produced in major amounts.
Although this process is very useful, it presents the disadvantage of being predicated upon homogeneous catalysis and concomitant problem of how to eliminate the acid catalyst at the end of the reaction. To eliminate the acid, it can be separated out [Ind. Eng. Chem. Prod. Res. Dev., 5, No. 3 (1976)] by aqueous washing and then the reaction medium can be treated with a mixture of water and isopropyl ether. The residual acid in the aqueous phase and the phenol and diphenols formed are extracted with isopropyl ether. It is then necessary to separate the isopropyl ether by distillation, followed by distillation of the phenol and diphenols.
FR-A-2,266,683 describes the hydroxylation of phenol in the presence of a ketone. Such process improves the yield of the reaction in respect of the hydroquinone and pyrocatechol. However, in all of the examples a larger amount of pyrocatechol is produced relative to that of hydroquinone.
And FR-A-2,667,598 describes a hydroxylation process wherein the amount of hydroquinone relative to the amount of pyrocatechol is increased and, in a preferred embodiment thereof, more hydroquinone than pyrocatechol is produced.
Said '598 process entails the hydroxylation of phenol in the presence of a catalytically effective amount of a strong acid, the reaction also being carried out in the presence of a ketone compound selected from among benzophenone and those benzophenones in which the hydrogen atoms of the aromatic nucleus are substituted by an electron-donor group.
In accordance with this process described in FR-A-2,667,598, the presence of the ketone compound during the hydroxylation of the phenol affects the selectivity of the reaction and hydroquinone/pyrocatechol ratios ranging from 1.0 to 1.13 are advantageously obtained.
However, such a process also presents the problem of eliminating the acid catalyst because it entails homogeneous catalysis employing a strong acid.